Catalysts comprising cobalt on a support such as silica or alumina are known in the art for hydrogenation reactions, e.g. for the hydrogenation of aldehydes and nitrites and for the preparation of hydrocarbons from synthesis gas via the Fischer-Tropsch reaction.
In comparison with other catalytic metals such as copper and nickel used for hydrogenation reactions, cobalt is a relatively expensive and so, to obtain the optimum activity, it is desirable that as much as possible of the cobalt present is in an active form accessible to the reactants.
For hydrogenation reactions, the active form of the cobalt is elemental cobalt although in the active catalyst only some, rather than all, of the cobalt is normally reduced to the elemental form. Hence a useful measurers the exposed surface area of elemental cobalt per g of total cobalt present. Except where expressly indicated, as used herein, total cobalt contents are expressed as parts by weight of cobalt (calculated as cobalt metal, whether the cobalt is actually present as the metal or is in a combined form, e.g. as cobalt oxides) per 100 parts by weight of the catalyst or precursor thereto.
Cobalt catalysts on different carriers are disclosed in “Stoichiometries of H2 and CO Adsorptions on cobalt”, Journal of Catalysis 85, pages 63-77 (1984) at page 67, table 1. From the total maximum H2 uptake, it is possible to calculate the cobalt surface area per gram of catalyst and the cobalt surface area per gram of cobalt.
U.S. Pat. No. 5,874,381 describes a cobalt on alumina catalyst which contains between 3 and 40% by weight of cobalt and which has a relatively high cobalt surface area of above 30 m2/g of total cobalt.
As indicated above, the dispersion of the cobalt on the carrier is important since it is the surface of the cobalt of the catalyst which is catalytically active. Therefore it is beneficial to maximise the surface area of the metal which is present so as to produce a catalyst which has a high cobalt surface area per unit mass of total cobalt. It may be expected that the dispersion of the cobalt on the catalyst would be maximised at relatively low loadings of cobalt and that, as the amount of cobalt contained in the catalyst is increased, the surface area per gram of cobalt would decrease because the cobalt becomes more difficult to disperse on the support.
The aforementioned U.S. Pat. No. 5,874,381 suggests and exemplifies the production of the catalysts by impregnation of shaped transition alumina particles, e.g. extrudates, with a solution of cobalt ammine carbonate, followed by removal of the excess solution and heating to decompose the cobalt ammine carbonate. We have found that the preparation of cobalt catalysts by the decomposition of cobalt ammine carbonate may be improved.